With a technology for coding a speech signal hierarchically, generally called scalable speech coding, if coded data of a particular layer is lost, the speech signal can still be decoded from coded data of another layer. Scalable coding includes a technique called band scalable speech coding. In band scalable speech coding, a processing layer that performs coding and decoding on a narrow-band signal, and a processing layer that performs coding and decoding in order to improve the quality and widen the band of a narrow-band signal, are used. Below, the former processing layer is referred to as a core layer, and the latter processing layer as an extended layer.
When band scalable speech coding is applied to speech data communications on a communication network in which the transmission band is not guaranteed and coded data may be partially lost or delayed, for example, the receiving side may be able to receive both core layer and extended layer coded data (core layer coded data and extended layer coded data), or may be able to receive only core layer coded data. It is therefore necessary for a speech decoding apparatus provided on the receiving side to switch an output decoded speech signal between a narrow-band decoded speech signal obtained from core layer coded data alone and a wide-band decoded speech signal obtained from both core layer and extended layer decoded data.
A method for switching smoothly between a narrow-band decoded speech signal and wide-band decoded speech signal, and preventing discontinuity of speech volume or discontinuity of the sense of the width of the band (band sensation), is described in Patent Document 1, for example. The speech switching apparatus described in this document coordinates the sampling frequency, delay, and phase of both signals (that is, the narrow-band decoded speech signal and wide-band decoded speech signal), and performs weighted addition of the two signals. In weighted addition, the two signals are added while changing the mixing ratio of the two signals by a fixed degree (increase or decrease) over time. Then, when the output signal is switched from a narrow-band decoded speech signal to a wide-band decoded speech signal, or from a wide-band decoded speech signal to a narrow-band decoded speech signal, weighted addition signal output is performed between narrow-band decoded speech signal output and wide-band decoded speech signal output. Patent Document 1: Unexamined Japanese Patent Publication No. 2000-352999